A converter for conversion between alternating current and high-voltage direct current, connected between an ac link and a dc link, generates, through its mode of operation, harmonics in the current in the ac network and in the voltage of the dc link. The currents associated with the latter voltage harmonics, and in particular the so-called ground-mode current, constitute a source of disturbance for telecommunication equipment located in the vicinity of the dc link. To eliminate these disturbances to the greatest possible extent, passive shunt filters are often used in the dc link, tuned to frequencies which are integer multiples of a product of the pulse number of the converter and the system frequency in the ac network, as well as designed as high-pass filters. These filters are generally not capable of completely extinguishing harmonics in the dc link. The reasons therefor are, for example, drift in the system frequency and in component values in the filters, but also the fact that the filter impedance at resonance frequency is not always negligible compared with the impedance in the other parts of the system. Further, during commutations and because of phase asymmetries in the ac network, harmonics of other frequencies than those mentioned above are normally also generated.
The passive filters are therefore nowadays often supplemented with active filters, which supply to the dc link an influencing quantity in the form of a current or a voltage for the purpose of reducing the amplitude of the harmonics which are generated by the phenomena described above. By measuring the remaining harmonic content in the direct current, the supplied current or voltage may thus be given such a form that, in principle, it fully eliminates the harmonic content. Such a filter comprises a power amplifier which is controlled by control equipment which is supplied with a sensed value of the harmonics, usually an actual current value formed in dependence on the harmonic current in the dc link. The control equipment forms, in dependence on a difference of a reference value for the harmonics and their sensed value, a control signal which is supplied to the power amplifier. In dependence on the control signal, the power amplifier generates the influencing quantity and supplies this to the dc link as one of a series quantity or a shunt quantity. Such filter circuits are exemplified in L. Gyugyi and E. C. Strycula: Active AC Power Filters, IAS 76 Annual, Paper 19-C. The document describes filter circuits for ac networks, but circuits which, in principle, are similar may also be applied to a dc link. In practice, the influencing quantity is usually supplied as a shunt quantity in that the power amplifier is connected in parallel with the disturbance source, in this case the converter.
The control signal is formed by a controller, which usually has a proportional or a proportional/integrating characteristic, which controller is supplied with the above-mentioned difference of the reference value for the harmonics and their sensed value.
An embodiment of such a controller is described in the published international patent application WO 93/09585. The controller forms the control signal in the form of a pulse train, repetitively with a cycle time corresponding to the repetitiveness of the disturbance source, in this case the time between two commutations in the converter, and comprises delay filters for adaptation of the controller to time delays in the transfer function for the external circuit connected to the controller.
The controller may also comprise filter devices for reducing or eliminating, in its output signal, certain frequency components or frequency bands, for example within a lower frequency range, the disturbing influence of which on the surroundings is low but which require a high power output from the power amplifier to be able to be eliminated.
The unpublished Swedish patent application 9700897-3 describes control equipment for an active filter, comprising at least one, and generally a plurality of, control units arranged like each other, each one for reduction of a tone of a tone frequency n.omega. of the ac components of the direct current. In this connection, tone means an interference signal, for example a harmonic current, of the tone frequency n.omega., where .omega. designates the system frequency of the alternating-voltage network and n, the ordinal number of the tone, a real number separate from zero, preferably an integer, n=n.sub.1, n.sub.2, . . . n.sub.m. Especially in installations for transmission of high-voltage direct current, where the converters included, because of their mode of operation, generate, on their direct-voltage sides, harmonics of the ordinal numbers q=kp to the system frequency of the alternating-voltage network, where p is the pulse number of the converter and k is a positive integer, the ordinal numbers may be chosen as integer multiples of the pulse number of the converter.
Each one of the control units is supplied with the actual current value and generates, in dependence thereon, an output signal. The control signal for the active filter is generated in dependence on this output signal, or in the event that the control equipment comprises a plurality of control units, in dependence on the sum of the outputs signals thereof, whereby a simultaneous reduction of the amplitude of a number of current harmonics of the tone frequencies n.omega.=n.sub.1 .omega., n.sub.2 .omega., . . . n.sub.m .omega. is achieved.
Each one of the control units comprises a first frequency-transforming member which forms values of amplitude and phase position for the tone which is associated with the respective control unit. These values are then processed separately in a separate controller with proportional/integrating characteristic, whereupon the output signals from the two controllers are summed and an output signal with a frequency equal to the tone frequency associated with the controller is recreated. The transfer function of the control unit is adapted, with respect to amplitude and phase position, to the transfer function for the external circuit connected to the control unit.
A converter station intended for bipolar operation essentially comprises two converters, which are series-connected on the dc side, and generates, in relation to ground, a direct voltage with positive polarity and a direct voltage with negative polarity (FIG. 1). The interconnecting point between the converters is connected to ground via an electrode line and a dc link, comprising two pole lines, is energized with the positive and the negative voltage, respectively. During bipolar operation, the current through the electrode line is zero or near zero and the power is transmitted via the pole lines. Only in exceptional cases, in case of a fault on or maintenance of a pole, the station is used in unipolar operation, in which case the dc circuit consists of one of the pole lines and the ground connection to an additional station connected to the pole line.
In converter stations of the above kind, an active filter is usually connected between each of the pole lines and ground, whereby the purpose of the control of the filters is to reduce the harmonic content in the ground-mode current. Each one of the pieces of control equipment of the filters is adapted to form the control signal in dependence on a sum of the harmonic currents of both pole lines, with the purpose of reducing this sum of harmonic currents. Studies have shown that, particularly in this operating case, where thus both pieces of control equipment are supplied with the same actual current values, interference phenomena may arise between the filters, characterized in that each one of the filters generates influencing quantities resulting in harmonic currents with a certain frequency content, which in turn the second filter strives to reduce by forming an influencing quantity with a corresponding frequency content. This phenomenon leads to an unstable control of the harmonic content in the dc link.